1. Field of the present invention
The present invention relates to a non-reciprocal circuit element, in particular, a non-reciprocal circuit element such as an isolator and a circulator, which is preferably for use in a microwave band, a module thereof, and a transmission and reception module including the non-reciprocal circuit element.
2. Description of the Related Art
Hitherto, a non-reciprocal circuit element such as an isolator and a circulator has characteristics such that it transmits signals in the predetermined specific direction only and does not transmit signals in the reverse direction. Using this characteristics, the isolator is used for a transmission circuit unit and a reception circuit unit of a mobile communication device such as a cellular phone, for example.
As the non-reciprocal circuit element (dual port-type isolator) of this type, an isolator as disclosed in Japanese Patent No. 4508192 has been known. The isolator is configured by an equivalent circuit as illustrated in FIG. 18. That is to say, a first center electrode 35 (inductor L1) and a second center electrode 36 (inductor L2) are arranged on the surface of a ferrite 32 so as to intersect with and be insulated from each other. A capacitor C1 and a resistor R that are connected to each other in parallel are connected between one end of the first center electrode 35 connected to an input port P1 and one end of the second center electrode 36 connected to an output port P2. Further, a capacitor C2 is connected in parallel with the second center electrode 36. Note that capacitors Cs1 and Cs2 are capacitors for matching of input and output impedances.
A dual port-side isolator configured by the equivalent circuit has characteristics as illustrated in FIG. 19. A curved line Y1 in FIG. 19 indicates a forward transmission characteristic (insertion loss characteristic) from the input port P1 to the output port P2 and a curved line Y2 indicates a reverse attenuation characteristic (isolation characteristic) from the output port P2 to the input port P1. The insertion loss has a characteristic of being low over a wide band as is obvious from the curved line Y1 but the isolation is achieved in a relatively narrow band as is obvious from the curved line Y2. Due to this, it becomes gradually difficult for the isolator to cope with a wide frequency band and a plurality of frequency bands that are used in mobile communication in recent years.
The isolator of this type is used for the transmission circuit unit. If it can be used for a transmission and reception circuit unit, the circuit configuration can be reduced in size.